Separations
\(\require{mhchem}\) Chemists are always being asked how to separate mixtures of things. A mixture is two or more substances put together but not chemically combined. We use different properties of the two substances to achieve separation.
Example: you are given a mixture of dry salt and sand. How can we separate them? If you had enough time, a microscope, and a very fine pair of tweezers, you could examined the grains one by one. The cubic grains would be salt, and you could (in theory) sort those out from the others. That would take a dreadfully long time, but this technique of hand picking can occasionally be useful. Louis Pasteur (inventor of Pasteurisation) used it to separate left- and right-handed crystals. In hand picking we are using a physical property of the substances—their appearance—to separate the two substances.
An easier way is to use a different property: solubility. If we add water to our mixture, the salt dissolves and the sand doesn't. We can then use a filter funnel and filter paper to separate the undissolved sand, which stays in the filter paper as the residue, from the salt dissolved in water, which is the filtrate. The process is called filtration.
See BBC Bitesize on Filtration for diagrams
How can we separate
Example: you are given a mixture of dry salt and sand. How can we separate them? If you had enough time, a microscope, and a very fine pair of tweezers, you could examined the grains one by one. The cubic grains would be salt, and you could (in theory) sort those out from the others. That would take a dreadfully long time, but this technique of hand picking can occasionally be useful. Louis Pasteur (inventor of Pasteurisation) used it to separate left- and right-handed crystals. In hand picking we are using a physical property of the substances—their appearance—to separate the two substances.
An easier way is to use a different property: solubility. If we add water to our mixture, the salt dissolves and the sand doesn't. We can then use a filter funnel and filter paper to separate the undissolved sand, which stays in the filter paper as the residue, from the salt dissolved in water, which is the filtrate. The process is called filtration.
See BBC Bitesize on Filtration for diagrams
How can we separate
| A solid | A liquid | A gas | |
|---|---|---|---|
| From a solid | If the two solids look different, hand picking. If only one is soluble in a solvent, dissolve, and filter. | Filtration | Gravity does the job for us. The solid will usually settle in a container with the gas above. If you need to do this quickly for a fine dust, there are many industrial machines that do the job. |
| From a liquid | See above right. | If, like oil and water, they don't mix, then we could pipette off the upper liquid, or use a separating funnel to run off the lower liquid. If they do mix, like ethanol and water, then distillation can be used; this relies on the liquids having different boiling points. At least \(\ce{25^{\circ}C}\) is recommended; the difference between ethanol (\(\ce{78^{\circ}C}\)) and water (\(\ce{100^{\circ}C}\)) is only \(\ce{22^{\circ}C}\), yet they can be reasonably well separated by simple distillation. If the difference is small, fractional distillation may be required. | In the simplest case, gravity will do the job for us. The liquid will settle at the bottom of a container with the gas above. Again, industrial machinery exists to do the job quickly and effectively. |
| From a gas | One way is to cool the gases until they liquefy and then use (fractional) distillation to separate the liquid, e.g. oxygen and nitrogen from the air can be separated this way. |
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